PROJECT SUMMARY/ABSTRACT Ischemia reperfusion injury (IRI) remains an inevitable component of organ transplantation and has been shown to enhance both acute and chronic rejection. IRI is largely mediated by neutrophils, which cause inflammation in response to innate immune pathways. It remains unknown exactly how the innate immune system causes neutrophil recruitment into transplanted organs. Preliminary data obtained in our laboratory suggests that toll-like receptor 3 (TLR3) and its downstream adaptor, TRIF, play a critical role in neutrophil adhesion and extravasation after cardiac transplantation. We aim to determine if TRIF expression on non-hematopoietic stromal cells is critical for neutrophil adhesion. Additionally, we will investigate whether or not TRIF regulates neutrophil recruitment through enhancement of necroptosis, a form of programmed cell death present during IRI that has been shown to adversely affect allograft survival. A previously established model of mouse heterotopic heart transplantation will be used to test our hypotheses. To control for expression of target proteins in transplanted hearts, we will generate bone marrow chimeras and utilize knockout mice as heart donors. Results will be analyzed using a combination of methods, including flow cytometry, ELISA, immunohistochemistry, blinded histology analysis, and survival curves. A novel imaging technology, known as intravital two-photon microscopy, will be used in multiple experiments to study leukocyte trafficking in real time. The results of our proposal will pave the way for targeted therapies that reduce IRI while maintaining immune pathways required for host defense again pathogens.